1. Field of the Invention
The present invention relates to a microswitch and a method of manufacturing the same, particularly, to a microswitch using a thin film made of a carbon series material and a method of manufacturing the same.
2. Description of the Related Art
In recent years, attentions are paid to a MEMS (Micro Electro Mechanical Systems) technology for manufacturing an integral structure comprising a fine mechanical structure and an electronic circuit by using a semiconductor fine fabrication technology. The MEMS technology is employed in various technical fields including, for example, a fine mechanical switch, hereinafter referred to as xe2x80x9cmicroswitchxe2x80x9d. The microswitch exhibits frequency characteristics which are more satisfactory than the semiconductor switch and, thus, is expected to be utilized in the field of telecommunications. Also, the microswitch can be miniaturized and integrated more easily than the conventional relay utilizing the electromagnetic force and, thus, is expected to be utilized in the field of vehicles.
As an example of the microswitch utilizing the particular MEMS technology, a microswitch manufactured by utilizing the Ni plating is disclosed in xe2x80x9cPaul M, Zavracky et al., Micromechanical Switches Fabricated Using Niker Surface Micromachiningxe2x80x9d, which is reported in xe2x80x9cJournal of Microelectro Mechanical Systems, (USA), (IEEE/IEE), 1997, Vol. 6, p. 3, FIG. 2xe2x80x9d.
The method of manufacturing the microswitch is schematically illustrated in FIG. 2 of the publication noted above, and the fabrication procedure is described in this publication with reference to FIG. 2. In the fabrication procedure disclosed in this publication, a silicon oxide film is formed first on a Si substrate, followed by forming on the silicon oxide film a Cr film and a Au film forming a first contact layer. Then, a source electrode, a gate electrode and a drain electrode are formed by the patterning using the photolithography. After formation of these electrodes, a Cu film acting as a sacrificial layer is formed, followed by etching the Cu layer so as to form a hemispherical concave portion and a hole extending to reach the source electrode. Then, a resist layer is patterned so as to form a Au film acting as a second contact layer and subsequently forming a beam by a Ni plating. Finally, the resist layer and the Cu layer acting as the sacrificial layer are removed so as to finish the manufacture of the device.
If a voltage is applied to the gate electrode in the microswitch of the construction described above, the beam is electrostatically deformed toward the substrate. If the voltage exceeds a certain value, the electrostatic force overcomes the elastic force of the beam so as to permit the hemispherical contact formed in the tip of the beam to be brought into contact with the drain electrode, with the result that the source-drain passageway is rendered conductive so as to form an xe2x80x9conxe2x80x9d state. If the voltage ceases to be applied to the gate electrode, the beam is brought back to the original state, with the result that the contact is moved away from the drain electrode so as to form an xe2x80x9coffxe2x80x9d state.
In general, Au is widely used as a material of the contact included in the microswitch. It should be noted in this connection that the on-resistance of the microswitch includes in general a contact resistance and a film resistance. The contact resistance is derived from the situation that the irregularity on the contact surface causes the actual contact area to be markedly smaller than the apparent contact area. On the other hand, the covering resistance is derived from the situation that the contact surface is covered with a thin insulating layer. In order to diminish the former one of the contact resistance, it is necessary to increase the contact force so as to enlarge the contact area, or to use a material that is likely to be deformed. On the other hand, in order to diminish the latter one of the covering resistance, it is necessary to increase the contact force so as to mechanical destroy the insulating layer on the surface, or to use a material on which an insulating layer is unlikely to be formed. However, an electrostatic force is used in general as a driving force in the microswitch. What should be noted is that the electrostatic force is very small. In general, the electrostatic force is capable of generating only about xcexcN to mN of the contact force. Such being the situation, Au, which can be deformed easily and on the surface of which an insulating film is not formed, is widely used as the material of the contact included in the microswitch.
However, the microswitch thus manufactured leaves room for further improvement in respect of its life. Particularly, Au used for forming the contact layer is known as a contact material that is likely to bring about a so-called xe2x80x9cstickingxe2x80x9d, which is the phenomenon that the both poles thereof are stuck to each other so as to make it difficult to permit the both poles to be separated from each other. It follows that Au used for forming the contact layer is said to leave a problem in respect of the reliability for a long time.
A microswitch using a diamond thin film is reported as a measure for overcoming the sticking problem noted above in, for example, an article entitled xe2x80x9cSurface micromachined diamond microswitchxe2x80x9d included in a magazine xe2x80x9cDiamond and Related Materials, Vol. 9, p.970, FIG. 2xe2x80x9d by S. Ertl, et al, which was published by xe2x80x9cElsevier Science, the Netherlandsxe2x80x9d in 2000xe2x80x9d.
The method of manufacturing a microswitch by using a diamond thin film is illustrated in FIG. 2 of the publication quoted above. According to the description relating to FIG. 2, an i-type diamond thin film forming an insulating layer is formed first on a Si substrate. Then, a p+-type diamond thin film having a high dopant concentration is formed, followed by patterning the p+-type diamond thin film so as to form a gate electrode and a first contact layer. Further, a SiO2 layer acting as a sacrificial layer is formed, followed by selectively etching the SiO2 layer so as to form a convex portion and a hole. After the selective etching step, a p+-type diamond thin film is formed, followed by pattering the diamond thin film so as to form a beam. In the beam forming step, a contact is formed at the tip of the beam. Finally, the sacrificial layer is removed so as to form a metal electrode, thereby finishing the manufacture of the device.
Likewise, a diamond microswitch structure is disclosed in, for example, an article entitled xe2x80x9cDiamond microwave microswitchxe2x80x9d by M. Adamschik, et al, which is included in a magazine xe2x80x9cDiamond and Related Materials, Vol. 11, p.672xe2x80x9cpublished by xe2x80x9cElsevier Science, the Netherlandsxe2x80x9d in 2002xe2x80x9d. Disclosed in this prior art is the structure that the current passageway starting from the metal electrode to reach again the metal electrode through the contact made of the p+-type diamond and the first contact layer is made as short as possible. It is reported that the particular structure makes it possible to decrease the resistance component derived from the bulk resistance of the diamond thin film.
Graphite, which is made of carbon like diamond, is known to be a material that does not bring about the sticking problem and, thus, was widely used in the past as a material of the sliding contact. It is reported by the research group referred to above that the sticking is not brought about in diamond, too, as reported in, for example, an article entitled xe2x80x9cSurface micromachined diamond microswitchxe2x80x9d included in a magazine xe2x80x9cDiamond and Related Materials, Vol. 9, p.970, FIG. 2xe2x80x9d by S. Ertl, et al, which was published by xe2x80x9cElsevier Science, the Netherlandsxe2x80x9d in 2000xe2x80x9d.
Diamond exhibits satisfactory mechanical characteristics, the thermal conductivity, and the corrosion resistance and, thus, makes it possible to achieve a microswitch capable of stably switching a large current over a long period of time.
The microswitch using a diamond thin film referred to above exhibits satisfactory mechanical characteristics, the thermal conductivity, and the corrosion resistance. In addition, the sticking problem is unlikely to be generated in the microswitch using the diamond thin film. It follows that the microswitch using the diamond thin film provides the possibility of realizing a microswitch capable of stably switching a large current over a long period of time and excellent in its reliability.
However, a serious problem is left unsolved in the device disclosed in the prior art publication referred to above, as pointed out below. Specifically, diamond is basically a semiconductor. The resistivity of a p+-type diamond (B-doped diamond) is at least 1,000 times as high as that of the ordinary metal. The on-resistance of the microswitch made of a metal is determined by substantially the contact resistance at the contact interface. However, the on-resistance of the microswitch made of diamond corresponds to the series resistance of the contact resistance and the bulk resistance of the diamond thin film, i.e., the sum of the contact resistance and the bulk resistance noted above. It follows that, even if the contact resistance is lowered by increasing the contact area by, for example, planarizing the contact surface, it is impossible to lower the on-resistance unless the series resistance is also lowered.
In the structure disclosed by S. Ertl et al, the thickness of the contact made of a p+-type diamond thin film is decreased so as to diminish the distance between the metal electrode on the back surface and the contact surface. However, this technology is limited in terms of the mechanical strength of the diamond thin film. Also, the diamond film is formed in general by a CVD method. In the initial stage of the CVD process for forming the diamond film, the diamond is in the form of isolated particles. Since a continuous film is formed in the stage that the diamond film is allowed to have a reasonable thickness, it is difficult to form a thin continuous film itself. Further, in the structure in question, the distance between the contact surface and the electrode corresponds to the distance in a direction parallel to the film. It follows that it is difficult to decrease the resistance component in the first contact layer.
Incidentally, the sticking problem is unlikely to take place in graphite, which is also the carbon-based material, as in diamond. However, the problem that the resistivity is several places higher than that of metal is inherent in graphite as in diamond.
An object of the present invention is to provide a microswitch having a low on-resistance, which utilizes the high reliability of a carbon-based material, and a method of manufacturing the particular microswitch.
According to an aspect of the present invention, there is provided a microswitch, comprising:
a substrate;
a first electrode portion fixed to the substrate and having a first contact surface, the first electrode portion including a first layer formed of a first carbon series material and provided with a group of first fine holes extending to reach the first contact surface, and first metal segments formed of a first metal material and buried in the first fine holes, respectively;
a second electrode portion arranged to face the first electrode portion with a gap provided therebetween and having a second contact surface movable toward the first electrode portion, the second electrode portion including a second layer formed of a second carbon series material and provided with a group of second fine holes each extending to reach the second contact surface, and second metal segments formed of a second metal material and buried in the second fine holes, respectively; and
a deformable structure configured to support the second electrode portion on the substrate to face the second contact surface to the first contact surface with a gap in a non-contact state, and configured to be deformed to shift the second electrode portion and contact the second contact surface with the first contact surface in a contact state.
According to an another aspect of the present invention, there is provided a method of manufacturing a microswitch, comprising:
forming a first electrode portion having a first contact surface on a substrate such that the first electrode portion is fixed to the substrate, the forming the first electrode portion including:
forming a first carbon series material layer on the substrate;
forming a first metal layer on the first carbon series material layer;
subjecting the first metal layer to an anodic oxidation within an acidic solution so as to form a first porous film in at least a surface region of the first metal layer;
subjecting the first carbon series material layer to an anisotropic etching with the first porous film used as a mask so as to form a group of first fine holes; and
burying a first metal material in each of the first fine holes;
forming a second electrode portion positioned to face the first electrode portion with a gap, the second electrode portion having a second contact surface movable toward the first electrode portion, the forming the second electrode portion including:
forming a sacrificial layer having a pattern on the substrate;
forming a second carbon series material layer on the sacrificial layer and on the substrate;
forming a second metal layer on the second carbon series material layer;
subjecting the second metal layer to an anodic oxidation within an acidic solution so as to form a second porous film in at least a surface region of the second metal layer;
subjecting the second carbon series material layer to an anisotropic etching with the second porous film used as a mask so as to form a group of second fine holes; and
burying a second metal material in each of the second fine holes; and
forming a deformable structure configured to support the second electrode portion on the substrate to face the second contact surface to the first contact surface with a gap in a non-contact state, and configured to be deformed to shift the second electrode portion and contact the second contact surface with the first contact surface in a contact state, the forming the deformable structure including removing the sacrificial layer.